VideoCommon: add milliseconds elapsed time value to pixel shaders as a uniform to be able to support animation effects in custom shaders

docs/CustomPipelineGraphicsMod.md

@@ -105,6 +105,7 @@ vec4 custom_main( in CustomShaderData data )
 |``tev_stages``               | CustomShaderTevStage[]  | v1    | An array of TEV stages, the amount is specified by ``tev_stage_count``                        |
 |``tev_stage_count``          | uint                    | v1    | The count of TEV stages                                                                       |
 |``final_color``              | vec4                    | v1    | The final color generated by Dolphin after all TEV stages are executed                        |
+|``time_ms``                  | uint                    | v1    | The time that has passed in milliseconds, since the game was started.  Useful for animating   |
 
 `CustomShaderLightData` is used to denote lighting data the game is applying when rendering the specific draw call.  It has the following structure:
 
@@ -247,4 +248,4 @@ vec4 custom_main( in CustomShaderData data )
 	}
 	return vec4(total_diffuse * vec3(0, 0, 1), 1);
 }
-```
+```